1. Field of the Invention
The invention relates to a process for the preparation of a laminate from at least one layer of a metal and at least one layer of graphite, wherein a non-detachable bond is provided over a whole surface between the metal and the graphite and the graphite which is used is prepared by pressing expanded graphite.
Laminates of metal plates or sheets and of graphite sheets or foils that are manufactured by compression of vermicular or expanded graphite are known (from U.S. Pat. No. 3,404,061; German Published, Non-Prosecuted Application DE-OS 25 18 351; U.S. Pat. No. 4,422,894; and company literature entitled SIGRAFLEX from the firm Sigri Great Lakes Carbon GmbH of Wiesbaden, Federal Republic of Germany). They are used mainly for flat gaskets, parts built into ovens, radiation shields, filter plates in electrostatic filters and corrosion-resistant linings.
The primary reason for the development of such laminates was the comparatively low resistance to tensile or bending loads shown by graphite sheets or foils manufactured by pressing expanded graphite. During handling in rough routine operations such low resistance often resulted in damage to the non-reinforced graphite elements, restricting the use of products of that type which otherwise had outstanding thermal, electrical and chemical properties.
The configuration and order of the individual layers in such laminates can be freely selected to a large extent and is dependent on the intended application. In most cases, the graphite is applied to one or both sides of the metal layer.
Two kinds of such laminates can be distinguished according to the nature of the bond between the layers of metal and graphite. In the first case, the bond is mechanical. The metallic part has surface structures which, when pressing the graphite with the metal part, either penetrate the graphite or into which the graphite penetrates by flow processes.
Examples thereof are tanged steel sheets, sheets with untrimmed holes, wire cloth, sintered metals or metal surfaces with porous, rough or damaged surfaces such as, for example, the surfaces of sealing flanges. A frequently undesirable bonding of that kind of flat seals with the opposing surfaces between which the seal is fixed is described, for example, in German Patent DE 32 44 595 C2 (column 2, lines 14 to 28), corresponding to UK Patent GB 2 131 500 B; and in German Published, Non-Prosecuted Application DE 37 19 484 A1 (column 1, line 68 to column 2, lines 1 to 8). Bonds of that type, which are not reproducible and do not appear uniformly distributed over the surfaces that are in contact, are observed only after long use of surfaces which are stretched together under sealing conditions and cannot therefore be used as a basis for the preparation of laminates from layers of metal and graphite of the kind described above.
In the second case, the metal and graphite surfaces are bonded together with a force-locking connection by adhesion using organic or inorganic bonding agents. A force-locking connection is one which connects two elements together by force external to the elements, as opposed to a form-locking connection which is provided by the shapes of the elements themselves. That method is used preferably in the presence of very smooth metal surfaces and/or if the surfaces cannot be provided with mechanical anchoring elements.
All bonds of the first and second types have disadvantages which restrict the use of or cause damage to the laminates.
In order to form a mechanically effective bond, relatively high surface pressures are required during the production of the laminate. Consequently, seals made from such material cannot be used for sealing tasks where only low tension forces are likely to be applied to the contact surfaces. In addition, the engagement or introduction of teeth or elevations of the metal reinforcement into the graphite or the sliding of graphite material into depressions in the metal reinforcement or into depressions in the opposing surfaces of a seal give rise in places to inhomogeneities and weaknesses in the graphite layer.
In seals, that results in inhomogeneities in the distribution of the surface pressure and consequently to the sealing action, in applications as coatings against aggressive media it leads to a reduction in the effective layer thickness and in high temperature applications it leads to uneven heating up or heat dissipation and consequently to distortions and warping resulting in separation in places. Separations are often unavoidable in that type Of bonding, in particular in the manipulation of laminate parts. That is particularly the case with thin and therefore bendable laminates or with small sections of such laminates. Such occurrences frequently lead to a part becoming useless or to the incidence of breakdowns during operation.
In seals with tanged steel reinforcement sheets, cracking of the metallic reinforcing sheets can moreover occur under load. Such damage is caused by points of tension which arise in the corners of the holes that have been formed by the teeth bending out from the sheets. There is also a requirement for the production of composite layers of metal and graphite free of bonding agents which do not have the disadvantages of reinforcements with tanged steel sheets.
The use of bonding agents to produce the bond between the layer or layers of metal and graphite has other disadvantages. Bonding agents of the kind known heretofore for the manufacture of typical laminates are applied in layer thicknesses of at least several .mu.m. This means that between the metal layer and the graphite layer there is an adhesive layer, the composition of which produces certain effects which must be considered, besides the adhesion. Such laminates can be used only at temperatures up to the decomposition temperature of the adhesive. Otherwise the consequence is defects, such as detaching of the graphite layer or blistering on the graphite layer, which prevent or reduce serviceability. Moreover, the penetration of the decomposition products from the bonding agent into apparatus or into the environment is often not tolerable. Even when the laminate section is clamped and consequently there is no fear of separation of the surfaces, as is the case with flat seals, the decomposition of the bonding agent, because of the decrease in volume associated with it, leads to a reduction of the sealing action through lowering of the prestressing due to the screws. Another damage mechanism arises in flat seals if adhesive layers soften through thermal or chemical action. The graphite layers, which are under compressive strain, then begin to slide laterally at relatively low surface pressures, which leads to tearing of the seal in extreme cases. However, in any case the sealing action is reduced because of the decrease in the prestressing due to the screws associated with the sliding process. Re-tightening the screws in that case cannot compensate for that defect, because the sliding processes continue. Finally, costly conductive adhesives must be used in applications such as filter plates in electric filters, for example, where good electrical conductivity is required perpendicularly to the surface of the laminate. Separations in the adhesive system, which can be caused by chemical reactions or by oscillations, in that case lead at least to a drastic reduction in efficiency.